Metal incorporation in sputter-deposited MoS 2 films studied by extended x-ray absorption fine structure
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Arun S. Bommannavar EXXON Research & Engineering Co., EXXON PRT, Bldg. 510E, Brookhaven National Laboratory, Upton, New York 11973 (Received 18 July 1994; accepted 14 April 1995)
Solid lubricant films produced by cosputtering metals with MoS 2 and by forming metal/MoS 2 multilayers are being planned for use in the next generation of solid lubricated devices on spacecraft, including gimbal and sensor bearings, actuators, and sliding electrical contacts. The films exhibit increased densities and wear lives compared to films without additives, but the mechanism of density enhancement is not well understood. The extended x-ray absorption fine structure (EXAFS) technique is ideal for elucidating the structure of these poorly crystalline films. We analyzed MoS 2 films cosputtered with 0, 2, and 10% Ni, as well as Ni/MoS 2 and Au(Pd)/MoS 2 multilayer films. The results obtained at the M o - K absorption edge showed that the metal-containing films comprised predominantly the same nanocrystalline phases present in similar films without added metals: pure MoS 2 and a M o S ^ O * phase. M o S ^ O * is isostructural with MoS 2 , with O atoms substituting for S atoms in the MoS 2 crystal lattice. For all Ni-containing films, EXAFS data obtained at the N i - K absorption edge showed that the Ni had not chemically reacted with the MoS2_xO^ and MoS 2 , but formed a disordered NiOx phase. However, Ni-cosputtered films showed decreasing M o - M o bond lengths in the MoSa^Ot phase with increasing Ni content, probably due to preferential oxidation of Ni compared to MoS 2 . EXAFS of these Ni-cosputtered films showed only a small decrease in short-range order with Ni content, while x-ray diffraction showed a concurrent large decrease in long-range order. The results indicate that film densification in Ni-cosputtered films is caused by NiO* formation at the edges of nucleating MoS 2 _ x O x /MoS 2 crystallites, limiting the crystallite size attainable within the films.
I. INTRODUCTION The addition of metals and other species is being explored as a way to enhance the performance of sputterdeposited MoS 2 lubricant/antiwear films in several applications on spacecraft. Cosputtered metal/MoS 2 films, produced by the simultaneous deposition of metal and MoS 2 , have been shown to exhibit desirable tribological characteristics. 1-5 Cosputtered films can be produced using separate targets or one compacted target. For example, rf-diode-deposited Au/MoS 2 films, grown from a single compacted target, displayed lower and more stable friction than pure MoS 2 films.1 In addition, less wear debris were produced for the cosputtered Au/MoS 2 film, which was attributed to its higher density. In another study, ten different metallic elements, as well as brass and bronze, were cosputtered with MoS 2 from separate metal and MoS 2 targets.2 Some of these metals
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to whom correspondence should be addressed. J. Mater. Res., Vol. 10, No. 8, Aug 1995
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performed better than pure MoS 2 films, with
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